1. Field of the Invention
The invention relates to an illumination system for a microlithography projection exposure system for illuminating an illumination field with the light from a primary light source.
2. Description of the Related Art
The efficiency of projection exposure systems for the microlithographic production of semiconductor components and other finely structured components is determined substantially by the imaging properties of the projection objectives. Furthermore, the image quality and the wafer throughput that can be achieved with the system are also determined substantially by properties of the illumination system arranged upstream of the projection objective. This must be capable of preparing the light from a primary light source, for example a laser, with the highest possible efficiency and, in the process, of producing the most uniform intensity distribution possible in an illumination field of the illumination system. Moreover, it should be possible to set various illuminating modes (settings) of the illumination system, in order for example to optimize the illumination in accordance with the structures of the individual patterns (e.g. on masks, reticles) to be imaged. Possible settings between different conventional settings with various degrees of coherence and also annular field illumination and dipole or quadrupole illumination are usual. The unconventional illumination settings for producing off-axis, oblique illumination can be used inter alia to increase the depth of focus by means of two-beam interference and also to increase the resolving power.
EP 0 747 772 describes an illumination system having a zoom-axicon objective, in whose object plane there is arranged a first diffractive raster element with a two-dimensional raster structure. This raster element is used to increase the geometric light guidance valve (or extendue) of the incident laser radiation slightly and to change the form of the light distribution in such a way that, for example, the result is an approximated circular distribution, annular distribution or quadrupole distribution. In order to change between these illuminating modes, first raster elements are interchanged. A second raster element, which is located in the exit pupil of the objective, is illuminated by the corresponding light distribution and forms a rectangular light distribution, whose form corresponds to the entry surface of a following rod-like light mixing element. By means of adjusting the zoom-axicon, the annularity of the illumination and the size of the area that is lit up (illuminated) can be adjusted.
EP 1 109 067 (corresponding to US 2001001247) describes an illumination system in which a changing device is provided for the optional changing of different diffractive optical elements in the light path of the illumination system. By interchanging the diffractive optical elements, various illuminating modes can be set. The system manages without a zoom-axicon module.
Other known possibilities for achieving off-axis illumination are shown, for example, in the U.S. Pat. No. 5,638,211, EP 500 393 B1 (corresponding to U.S. Pat. No. 5,305,054), U.S. Pat. No. 6,252,647 or U.S. Pat. No. 6,211,944.
In the case of illumination systems which operate with interchangeable optical elements (for example diffractive optical elements or spatial filters) in order to set different illuminating modes, the number of different illumination settings is limited by the number of different changeable elements. Appropriate changing devices can be constructionally complex.
DE 199 44 760 A1 discloses an illumination device for printing plates which permits modulation of the illumination intensity in the integrated digital screen imaging process (IDSI). In this case, the light from a light source falls onto a digital light modulator having a two-dimensional array of cells, which can be activated and deactivated via a computer-controlled system in order to deflect a specific pattern onto a light-sensitive substrate, which is moved relative to the light modulator. In one embodiment, the light modulator comprises a micro mirror arrangement (digital mirror device, DMD) having a large number of individual mirrors that can be driven individually. During printing, those mirrors which are not used for the exposure of the light-sensitive material are tilted in such a way that they deflect the light beam falling on them away from light-sensitive material. By means of the control system, the number of individual mirrors used in the exposure is thus changed. A similar system is disclosed by WO 00/36470.